Salicylic acid is a phenol as well as a carboxylic acid. It can therefore undergo two different types of esterification reactions, creating an ester either with the hydroxyl or with the acid. In the presence of acetic anhydride, acetylsalicylic acid (aspirin or ASA) is formed. Correspondingly, an excess of methanol will form methyl salicylate, which is also an analgesic. In this experiment, we shall use the former reaction to prepare aspirin.
Salicylic acid will not react significantly with acetic acid to produce aspirin. Acetic acid anhydride, however, is more reactive than acetic acid because the acetoxy group (-O2CCH3) is a much better leaving group than the OH- of acetic acid.
The reaction has one complication, however, in that an esterification can occur between the phenol and acid portion of adjacent salicylic acid molecules. Further, more molecules can bind to the remaining free substituents on these molecules to create a macromolecule, or polymer. The polymer is formed as a by-product.
Acetylsalicylic acid will react with sodium bicarbonate to form a water-soluble sodium salt, whereas the polymer remains insoluble. This difference can be used to purify the aspirin product.
The most likely impurity in the final product is salicylic acid, which can be either unconsumed reactant, or the result of hydrolysis of the aspirin product. Salicylic acid is removed during the purification steps as well. Salicylic acid, like most phenols, forms a highly colored complex with ferric chloride, and is easily detected. Aspirin does not form the colored complex because the hydroxyl has been acetylated.
1. Weigh 2.0 g of salicylic acid crystals, and place them in a 125-ml Erlenmeyer flask. (If mixture solidifies completely, proceed to step 2.)
2. Add 5 ml of acetic anhydride and 5 drops of...